Analog inputs: How to wire analog inputs


ACE are available with some number of general analog inputs.

ACE analog input interfaces are available for either 0-5 VDC, 0-10 VDC or 0-20 mA.

Analog inputs are normally used to connect to sensors outputs. Such sensors measure some physical parameter, such as pressure, temperature, liquid level, position, pH level or other such continuously variable measurement.

The sensors signal output should be connected to a signal input on the analog port and the sensors return or ground reference line must be connected to the PLC ground, next to signal 1 (or otherwise connected to the PLC’s ground).

ACE with current analog input ports (part number ending in ‘c’) should be used for analog current signals between 0 and 20 mA. The two most common type of current signals are 4-20 mA and 0-20 mA. Full range analog signals will convert to a value between 0 and 4095 (12 bits). For 4-20mA inputs, the converted value will be between 820 and 4095. The Scale function in vBuilder can be used to automatically convert the signal value to meaningful data.

For ACE that have a full 8 position port dedicated to analog inputs, the pin next to the sixth input is connected to the PLC’s internal 5V, through a 10 ohm resistor and a diode. This is provided to power a future analog input simulator. It servers no other purpose and should remain unconnected for application use. Analog input ports that do not occupy the full 8 position port do not have this connection.

The 0-20mA analog inputs have a 100 ohm resistor between the input and ground.  For the 0-10V inputs, its 1Mohm.  For the 0-5V inputs, its 499 Kohms.

About 0-20 mA analog inputs

Internally, we convert the 0-20/4-20mA signal to a 0-2V signal by use of a 100 ohm resistor to ground.
That means that the transistor ground and the transducer ground must be the same ground.
If you want to connect the same signal to multiple devices, it won’t work, unless the PLC is the last device in the chain and every other devices can handle the pass through.

Voltage for 0/4-20 mA inputs

It doesn’t matter what voltage the transducer (sensor) is powered from . Actually 24V is the most common. So long as the current is 4-20mA, everything is good.

Why am I measuring 1.9V on the analogue input?

The analog inputs go through an amplifier, to scale the voltage to the range the A/D converter works with, before being converted by the A/D converter. One feature of an amplifier is an offset voltage that keeps the signal balanced to achieve the desired output range. The connection between the analog input and the amplifier’s offset voltage in between 316K and 1Mohm, depending on whether it is a 4-20mA (316K), 0-5V (499K) or 0-10V (1M) input. If nothing is connected to the input, you will see the offset voltage on the 0-5V and 0-10V analog inputs, on all of our PLCs.

This will not affect an actual analog input signal from a transducer. The input resistance is so high that the actual input signal overwhelms this offset voltage.

How to convert current 4-20 mA output from a sensor to 0-10 V voltage using resistor ?

See this page

How to change the current of the 0-20 mA input using a simple potentiometer?

You can use a 1 kOhm resistor (R1) and a 1 kOhm potentiometer (R2).
Internally in the ACE, the current is converted to voltage (VM1 = 0-2 V) via a 100 oHm resistor (Rin).

About Ground loops

Don’t share grounds and don’t ground remote sensors
In the correct schematic above, each sensor has its own controller ground and is only grounded to a controller ground. In the incorrect schematic above, the sensor is grounded at a remote grounding point as well as to a controller ground
Signal Interference From Magnetic Induction

The primary sources of these noise problems are Magnetic Induction and Ground Imbalance.

Ground Loop Magnetic Field

Signal Interference From Ground Imbalance

If a large current is flowing in the ground system, and a sensor is placed in a circuit with a ground that also has a ground loop, then the voltage difference between the two ground points will be added to the signal.

Ground Imbalance (left) and Sensor Voltage and Ground Loop Voltage





See also More details about the analog input signal of ACE PLCs